Young, hotheaded stars could host habitable worlds

Half the battle in finding alien life is knowing where to look – and we might be missing some key places. Fresh research has found that planets orbiting hotheaded young stars may have the right stuff for life, increasing the number of places in the universe with that possibility.

A planet is considered habitable when it is small and dense enough to have a rocky surface, and the right distance from its star to host liquid water – a region dubbed the “habitable zone“.

But stars don’t have the same temperature throughout their lives. In their youths, before they reach a stable rate of nuclear fusion, stars burn hotter and up to 180 times as brightly as their mature counterparts, meaning their habitable zones are further away.

Because this fiery phase is temporary, astronomers had so far only considered the habitability of planets orbiting mature stars. But now, Lisa Kaltenegger and Ramses Ramirez at Cornell University in Ithaca, New York, suggest that some of these young hotheads could host life-bearing planets after all.

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The smallest and coolest stars, called M dwarf stars, can stay youthful for 2.5 billion years. That may be enough time for life to develop and then go underground or underwater to survive as the star cools.

“We call them ‘infant Earths’ because this will happen really early on in planetary evolution,” explains Kaltenegger. “It’s a really simple idea but nobody seems to have ever thought about it.”

Desiccated deserts?

Kaltenegger hopes that ground-based observatories such as the European Extremely Large Telescope in northern Chile – due to be operational by 2024 – will be powerful enough to detect a “wobble” indicating a nearby planet in stars of this smaller size.

However, planets orbiting close to young stars may lose their surface water as the star reaches its peak brightness – meaning planets that appear to be in the habitable zone are actually desiccated deserts. It is still possible that the lost water could be replaced during a period of “heavy bombardment” by asteroids, similar to what may have occurred on Earth.

“It’s interesting that planets could lose all their water initially,” says Sara Seager at the Massachusetts Institute of Technology in Cambridge. “Planetary systems are so complex that this is something people haven’t worried about before.”

However, Seager notes that the usefulness of the idea rests on whether or not nearby stars with these characteristics can be observed. “People like us [observers] want that list of target stars. It would take me much longer to go and figure out which stars are applicable here.”